DCS usually adopts a hierarchical and progressive structure, each level is composed of several subsystems, and each subsystem achieves a number of specific finite goals, forming a pyramid structure. Examining the hierarchical structure of DCS, DCS level and control management level are the two most basic links that make up DCS.
The process control stage specifically realizes the decentralized control functions such as signal input, conversion, operation and output. In different DCS, the control devices of the process control level are different, such as process control unit, field control station, process interface unit, etc., but their structural form is roughly the same, and can be collectively referred to as field control unit FCU. The process management level is composed of an engineer station, an operator station, a management computer, etc., to complete the centralized monitoring and management of the process control level, which is usually called an operation station. The hardware and software of DCS are designed according to the modular structure, so the development of DCS is actually to combine the various basic modules provided by the system into a system according to the actual needs, and this process is called system configuration.
1. On-site control unit
The field control unit is generally far away from the control center and installed close to the site, and its highly modular structure can be configured into a process control unit of varying sizes from a few monitoring points to hundreds of monitoring points according to the needs of process monitoring and control.
The structure of the field control unit is composed of many functionally dispersed plug-in boards (or cards) installed in the plug-in box according to a certain logical or physical sequence, and each field control unit and its control management level are connected by bus to realize information interaction.
The hardware configuration of the field control unit needs to complete the following:
Configuration of plug-ins Configure hardware devices such as host plug-ins (CPU plug-ins), power plug-ins, I/O plug-ins, and communication plug-ins according to the requirements and control scale of the system;
Hardware redundancy configuration Redundant configuration of key equipment is an important means to improve the reliability of DCS, DCS can usually achieve redundant configuration of host plug-ins, power plug-ins, communication plug-ins, networks, and key I/O plug-ins.
Hardware Installation Different DCSs have corresponding provisions in the logical order or physical order for the installation of various plug-ins in the plug-in box. In addition, the field control unit is usually divided into two types: basic type and extended type, the so-called basic type is that various plug-ins are installed in a plug-in box, but more often the need for an extensible structural form, that is, a field control unit also comprises a plurality of digital input/output expansion units, which are connected by bus to each other.
In essence, the structure and configuration requirements of the field control unit are consistent with the hardware configuration of the modular PLC.
2. an operation station
The operation station is used to display and record the process data from each control unit, and is the operation interface for the interaction of human and production process information. Typical operation stations include host systems, display devices, keyboard input devices, information storage devices and printout devices, etc., which mainly realize powerful display functions (such as analog parameter display, system status display, multiple screen displays, etc.), alarm functions, operation functions, report printing functions, configuration and programming functions, etc.
In addition, the DCS operation station is divided into an operator station and an engineer station. From the perspective of system functions, the former mainly realizes general production operations and monitoring tasks, and has functions such as data acquisition and processing, monitoring screen display, fault diagnosis and alarm. In addition to the general functions of the operator station, the latter should also have functions such as the configuration of the system, the modification of control targets, etc. From the perspective of hardware equipment, the engineer station and operator station of most systems are combined and distinguished by a single engineer keyboard.
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